A stand-off is the distance between a torch and a work to be processed in plasma processing. Maintaining the stand-off at an optimum value is selected as an objective in order to improve processing quality. A conventional stand-off control apparatus for plasma processing machines will be briefly described below.
A technique has been known (refer to, for example DE2706232) which tries to maintain an optimum stand-off h.sub.0 by monitoring an arc voltage V from the fact that "a stand-off h is in proportion to the arc voltage V at a fixed cutting speed F." However, the fixed stand-off system cannot cope with a case where the cutting speed F needs to be varied for improved processing accuracy and productivity. In other words, the fixed stand-off system can manipulate two-dimensional processing machines like XY tables with less difficulty, but basically cannot manipulate three-dimensional processing machines, which cover a wider range of cutting speeds.
In this connection, the applicants for the present invention have previously proposed a technique (refer to Japanese Patent Application No. 3-110790, published as Japanese Published Unexamined Patent Application (A) 5-378) of maintaining the optimum stand-off h.sub.0 by monitoring the arc voltage and the processing speed F, wherein the fact that "the arc voltage V is substantially in inverse proportion to the cutting speed F", as shown in FIG. 5, is incorporated into the aforesaid technique. The proposed technique is hereinafter referred to as speed-corrected stand-off system.
The correction of a target voltage V.sub.O for a speed in the speed-corrected stand-off system will now be described with reference to FIG. 5. In the figure, measurements of the cutting speed F and the arc voltage V are plotted for stand-off's h.sub.1 .about.h.sub.5 (h.sub.1 &lt;h.sub.5). For the stand-off's h.sub.1 .about.h.sub.5, as the cutting speed F increases, the arc voltage V drops substantially in an inversely proportional manner. This is because as the cutting speed F increases, the main anode point of the work comes closer to the torch. For example, with a reference cutting speed taken as F.sub.L and a target arc voltage as V.sub.L at the initially set optimum stand-off h.sub.1, when the cutting speed increases to F.sub.H, the arc voltage drops from V.sub.L to V.sub.H. However, because of the target arc voltage V.sub.L being fixed, the result of their comparison, i.e. V.sub.H &lt;V.sub.L, causes the torch to rise from h.sub.1 to h.sub.3 with a resultant failure to maintain the optimum stand-off h.sub.1. Hence, by correcting the initial target arc voltage V.sub.O for an inputted cutting speed F, i.e., by making correction to a new target arc voltage V.sub.H at the stand-off h.sub.1 and the cutting speed F.sub.H, the optimum stand-off h.sub.1 is maintained even when the cutting speed F varies. Accordingly, while the processing speed is kept unchanged at a stationary level before and after it varies, the quality of processing is quite good.
However, even in the speed-corrected stand-off system, when the processing speed varies, i.e., when variations in the processing speed begin, are under way, and then end, it is difficult to maintain the optimum stand-off h.sub.0 because variations in the arc voltage V get delayed due to delay in the response of the position of the plasma arc's main anode point to operations of a processing machine. The time of variations in the processing speed is usually short, but even so, there arises a problem that the quality of processing deteriorates during the time.